FIG. 1 depicts prior art. A hot-water supply apparatus shown in FIG. 1 is provided with a thermal fuse 3' enclosed in a glass tube or in a ceramic tube and connected to the inside of the rear of the shell. As illustrated, the thermal fuse 3' is located in close proximity to a heat exchanger 2' and to a combustion chamber 1' so that the thermal fuse 3' melts when the heat exchanger 2' or the combustion chamber 1' has reached an excessively high temperature. The thermal fuse 3' is electrically connected to a combustion control circuit (not shown). Although not shown, there is also prior art where plural thermal fuses are provided in scattered positions which are in close proximity to the combustion chamber and to the heat exchanger.
A combustion apparatus, especially a hot-water supply apparatus, is often used continuously for a long time. Continuous use of a combustion apparatus may cause its combustion chamber or heat exchanger to reach such a high temperature as to be cracked or perforated. If it is cracked or perforated, the combustion gas may leak through the crack or perforation and cause a fire. Thus, if the combustion chamber or the heat exchanger has reached such a dangerous temperature, it is necessary to detect it instantly and stop the combustion without delay.
In the prior art of FIG. 1, however, if a portion of the combustion chamber 1' or the heat exchanger 2' which is at a distance from the thermal fuse 3' has reached such a dangerous temperature, the thermal fuse 3' does not detect the dangerous temperature, or does not melt. In such a case, therefore, the thermal fuse 3' cannot prevent a fire. Plural thermal fuses provided in scattered positions around the combustion chamber and the heat exchanger are more successful in detecting a dangerous temperature than the single thermal fuse 3'. However, even plural thermal fuses in scattered positions do not always succeed in detecting a dangerous temperature since "blind spots" may exist. Also, it takes much time and trouble to provide plural thermal fuses in scattered positions. It is also expensive to provide plural thermal fuses.